Pulmonary alveolus

The thin-walled saclike terminal dilation of the respiratory bronchioles, alveolar ducts, and alveolar sacs, and from where gas diffusion between the air and blood occurs

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Pulmonary alveoli are the thin-walled saclike terminal dilations of the respiratory bronchioles, alveolar ducts, and alveolar sacs across which gas exchange occurs between alveolar air and the pulmonary capillaries. They occur in mammalian lungs since other vertebrates have different structures that are involved in gas exchange.1 In human lungs, there are about 700 million alveoli that produce 70m2 of surface area.2

The pulmonary alveoli consist of three major cell types. They are the type 1 alveolar cells, the type 2 alveolar cells, and the alveolar macrophages. Type 1 and type 2 alveolar cells are involved in the gas exchange whereas the alveolar macrophages are associated with phagocytosis, homeostasis, and tissue remodeling. The type 1 alveolar cells (or the squamous alveolar cells) cover most of the alveolar surface, i.e. about 90-95%. The type 2 alveolar cells (or the great alveolar cells) are cells secreting pulmonary surfactant. The surfactant is essential in decreasing the surface tension of water within the alveoli. It allows the membrane to separate, thus, improving gas exchange.